Hello John.
I had the same problem an I have found the solution in the page of "simple KPI" in GitHub:
https://github.com/alner/qsSimpleKPI

Here is an example:

@import url('https://fonts.googleapis.com/css?family=Indie Flower');
@import url('https://fonts.googleapis.com/css?family=Fredoka One');

& .label * {
font-family: 'Fredoka One';
font-size: 300%;
}

& .statistic-1 .value * {
background-color: green;
font-family: 'Indie Flower', sans-serif;
color: white !important;
}

& .value * {
background-color: yellow;
font-family: 'Indie Flower', sans-serif;
font-size: 900%;
color: red !important;
}

I hope this will be useful for you.